Work of Breathing and Pressure-Time Product on Pressure Support Ventilation,Continuous Positive Airway Pressure,and T-Piece

Abstract

BACKGROUND: After prolonged mechanical ventilation (MV), weaning may include gradual unloading of respiratory muscles, followed by trials of CPAP or spontaneous breathing on a T-piece. We compared the total work of breathing (WOB) and pressure-time product (PTP) on mechanically ventilated patients during: (1) 5 cm H2O of pressure support ventilation (PSV5); (2) 5 cm H2O CPAP using continuous-flow via an adjustable Downs flow generator (DFG) with an external PEEP valve (CPAPCF-5); (3) 5 cm H2O CPAP using the Siemens Servo 900C with a demand-valve system (CPAPDV-5); and (4) T-piece. METHODS: Measurements of WOB and PTP were obtained in 23 medical patients deemed ready for liberation from MV by standard clinical criteria, who had been ventilated for respiratory failure of various etiologies. WOB and PTP determinations were made for 5-minute intervals with a CP-100 Pulmonary Monitor and then analyzed using a Gaussian-breath-elimination method to omit breaths with artifactual changes in esophageal pressure caused by swallowing, coughing, and other non-ventilatory maneuvers. RESULTS: All patients were successfully liberated from MV. WOB in J/L: PSV5 = 0.89, CPAPCF-5 = 0.98, CPAPDV-5 = 0.99, and T-piece = 1.07. PTP in cm H2O · s · min−1: PSV5, = 197, CPAPCF-5 = 221, CPAPDV-5 = 222, and T-piece = 228. By ANOVA, WOB was comparable among the various modes and T-piece (p = 0.77); similar results were obtained for PTPs (p = 0.74). CONCLUSION: Although the mechanism of maintaining positive airway pressure is different with PSV5, CPAPCF-5, and CPAPDV-5, our study shows that the WOB and the PTPs are comparable with that of the T-piece, a spontaneous breathing mode. If the patient's ventilatory demands are met adequately, then low-level positive pressure and T-piece are equally effective for not developing respiratory muscle fatigue. Thus, following gradual weaning with PSV, when ventilatory demands are met and respiratory muscle performance is good, trials with T-piece or 5 cm H2O of CPAP, instead of 5 cm H2O of PSV on the Siemens Servo 900C, may offer no benefit prior to extubation or decannulation. To our knowledge, this is the only clinical study that demonstrates that at 5 cm H2O, WOB and PTP on the Siemens Servo 900C in the CPAP mode are comparable to those of a continuous-flow CPAP system using an adjustable DFG with a non-reservoir circuit and an external PEEP valve.

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